The present invention relates generally to bulk acoustic wave resonators and filters and, more particularly, to the tuning of such resonators and filters.
It is known that a bulk acoustic-wave (BAW) device is, in general, comprised of a piezoelectric layer sandwiched between two electronically conductive layers that serve as electrodes. When a radio frequency (RF) signal is applied across the device, it produces a mechanical wave in the piezoelectric layer. The fundamental resonance occurs when the wavelength of the mechanical wave is about twice the thickness of the piezoelectric layer. Although the resonant frequency of a BAW device also depends on other factors, the thickness of the piezoelectric layer is the predominant factor in determining the resonant frequency. As the thickness of the piezoelectric layer is reduced, the resonance frequency is increased. BAW devices have traditionally been fabricated on sheets of quartz crystals. In general, it is difficult to achieve a device of high resonance frequency using this fabrication method. When fabricating BAW devices by depositing thin-film layers on passive substrate materials, one can extend the resonance frequency to the 0.5-10 GHz range. These types of BAW devices are commonly referred to as thin-film bulk acoustic resonators or FBARs. There are primarily two types of FBARs, namely, BAW resonators and stacked crystal filters (SCFs). An SCF usually has two or more piezoelectric layers and three or more electrodes, with some electrodes being grounded. The difference between these two types of devices lies mainly in their structure. FBARs are usually used in combination to produce passband or stopband filters. The combination of one series FBAR and one parallel, or shunt, FBAR makes up one section of the so-called ladder filter. The description of ladder filters can be found, for example, in Ella (U.S. Pat. No. 6,081,171). As disclosed in Ella, an FBAR-based device may have one or more protective layers commonly referred to as the passivation layers. A typical FBAR-based device is shown in FIGS. 1a and 1b. As shown in FIG. 1, the FBAR device comprises a substrate 110, a bottom electrode 120, a piezoelectric layer 130, and a top electrode 140. The FBAR device may additionally include a membrane layer 112 and a sacrificial layer 114, among others. The substrate can be made from silicon (Si), silicon dioxide (SiO2), Gallium Arsenide (GaAs), glass or ceramic materials. The bottom electrode and top electrode can be made from gold (Au), molybdenum (Mo), tungsten (W), copper (Cu), nickel (Ni), titanium (Ti), Niobium (Nb), silver (Ag), tantalum (Ta), cobalt (Co), or aluminum (Al). The piezoelectric layer 130 can be made from zinc oxide (ZnO), zinc sulfide (ZnS), aluminum nitride (AlN), lithium tantalate (LiTaO3) or other members of the so-called lead lanthanum zirconate titanate family. The passivation layer is typically made from a dielectric material, such as SiO2, Si3N4, or polyimide, to serve as an electrical insulator and to protect the piezoelectric layer. It should be noted that the sacrificial layer 114 in a bridge-type BAW device is, in general, etched away in the final fabrication stages to create an air interface beneath the device. In a mirror-type BAW device, there is an acoustic mirror structure beneath the bottom electrode 120. The mirror structure consists of several layer pairs of high and low acoustic impedance materials, usually quarter-wave thick. The bridge-type and the mirror-type BAW devices are known in the art.
The desired electrical response in an FBAR-based device is achieved by a shear or longitudinal acoustic wave propagating in the vertical thickness through the device. Besides these wave modes, there exist other modes, including other shear modes, extensional modes and their higher harmonics. However, with respect to the operation point, the Lamb wave modes in the nearby frequencies are the unwanted spurious modes that may deteriorate the electrical response. In quartz crystals, the strength of these spurious modes is controlled by adjusting the thickness and the width of the top electrode. In an FBAR-based device, the dimension in thickness direction is so small that it renders thickness adjustment difficult and impractical. A possible solution to resolving the problems associated with the spurious modes is to thicken the edge of the top electrode. As disclosed in Kaitila et al. (WO 01/06647 Al, hereafter referred to as Kaitila), a frame-like structure 150 is formed on top of the top electrode 140 to thicken the edge thereof. As shown in FIGS. 1a and 1b, the frame-like structure 150 is a rectangular frame for defining a first zone and a second zone for acoustic wave excitation. The first zone is the area under the frame-like structure 150, and the second zone 148 is the area surrounded by the frame-like structure 150. With such a structure, the cut-off frequency of the piezoelectrically excited wave modes in the first zone and that in the second zone is different. When the width of the frame-like structure and the acoustic properties of the layer structure are properly arranged, the displacement relating to the strongest of the piezoelectrically excited resonance modes is substantially uniform in the second zone. The electrical response of an FBAR-based device with a thickened edge (solid line) and that without a thickened edge (dashed line) are presented on a Smith Chart as shown in FIG. 4.
As it is known in the art, a Smith Chart is a polar plot of the complex reflection, which represents the ratio of the complex amplitudes of the backward and forward waves. The Smith Chart helps translating the reflection coefficient into impedance, and it maps part of the impedance plane onto a unit circle. In an FBAR-based resonator, the piston mode is a vibration mode where the vibration amplitude is practically uniform over the second zone. If a resonator exhibits the piston mode, the spurious modes become very weakly excited and the response of the resonator is optimized with respect to the spurious resonances. In general, when the Smith Chart shows a clean circle, the structure of the resonator is close to a piston mode producing structure. Thus, the Smith Chart is a good indicator of the quality of the resonator response. In FIG. 4, the outermost circle that touches the square frame of the plot is the unit circle in the Smith Chart.
It should be noted that, as disclosed in Kaitila, the frame-like structure may be circular, square, polygonal, regular or irregular. Also, the frame-like structure can have different configurations, as shown in FIGS. 2 and 3, to achieve the piston mode. As shown in FIGS. 2 and 3, part of the piezoelectric layer 130 is covered by a passivation layer 160, and part of the passivation layer is sandwiched between the piezoelectric layer 130 and the frame-like structure 150 extended upward from the edge of the top electrode 140. In FIGS. 2 and 3, the frame-like structure 150 is basically where the top electrode 140 overlaps with the passivation layer 130. It should be noted that FIG. 1a is a cross sectional view of a BAW device, as viewed in the lateral direction, while FIG. 2 and FIG. 3 are cross sectional views of a BAW device, as viewed in the horizontal direction.
In FBAR-based ladder filters, the frequency of the shunt resonators must be down-shifted by adding an extra thin-film of a suitable material to the film stack of the resonator. The added thin-film is usually referred to as the tuning layer. The thickness of the tuning layer is determined by the desired frequency shift and is generally much smaller than the thickness of other layers on the device. If the shunt resonator in a ladder filter is designed to operate optimally, with regard to the suppression of the spurious mode without the tuning layer, adding the tuning layer may degrade the performance of the resonator by re-introducing the spurious resonance frequencies.
Thus, it is advantageous and desirable to provide a method of tuning the shunt resonator for frequency down-shifting without substantially degrading the performance of the shunt resonator.
It is a primary object of the present invention to provide a method for tuning a frequency down-shifted bulk acoustic wave device for reducing spurious resonance due to the frequency down-shifting, wherein a tuning layer formed on top of the top electrode of the bulk acoustic wave device is used to lower the resonant frequency of the device. This object can be achieved by configuring the tuning layer.
Thus, according to the first aspect of the present invention, a method of tuning a bulk acoustic wave device comprising a substrate and a plurality of acoustic wave generating and controlling layers formed on the substrate, wherein the acoustic wave generating and controlling layers include a piezoelectric layer formed between a first electrode and a second electrode for generating piezoelectrically excited acoustic signals, wherein the first electrode has a frame-like structure at an edge portion of the first electrode for reducing spurious resonance components in the acoustic waves, and the frame-like structure surrounds a center zone having a surface area, and wherein the device has a resonance frequency which can be down-shifted by modifying the first electrode. The method comprises the step of providing a tuning layer on top of the first electrode for modifying the first electrode such that the tuning layer covers at least the surface area of the center zone for reducing spurious resonance resulting from the tuning layer.
Alternatively, the tuning layer also covers at least part of the frame-like structure.
Preferably, the tuning layer substantially covers the entire frame-like structure.
According to the second aspect of the prevent invention, a bulk acoustic wave device comprising a substrate and a plurality of acoustic wave generating and controlling layers formed on the substrate, wherein the acoustic wave generating and controlling layers include a piezoelectric layer formed between a first electrode and a second electrode for generating piezoelectrically excited acoustic signals, wherein the first electrode has a frame-like structure at an edge portion of the first electrode for reducing spurious resonance components in the acoustic signals and the frame-like structure surrounds a center zone having a surface area, and wherein the device has a resonance frequency which can be down-shifted by modifying the first electrode. The device comprises a tuning layer provided on top of the first electrode for modifying the first electrode such that the tuning layer covers at least the surface area of the center zone, and alternatively, at least part of the frame-like structure as well as the surface area of the center zone. Preferably, the tuning layer substantially covers the entire frame-like structure as well as the surface area of the center zone.
The present invention will become apparent upon reading the description taken in conjunction with FIGS. 5 to 9.